When Gretchen Alfonso, a 29-year-old mother of three, was growing up in Erie, Pennsylvania, fishing was part of the culture. The locals knew that the perch from Lake Erie was contaminated with toxic pollutants like mercury, but fish-fry dinners were a tradition no one wanted to give up. So on Friday nights, people ate perch. "The grown-ups joked about how you shouldn't eat too much of it," Alfonso remembers. Everyone ate the fish, including Alfonso.

Pregnant women are regularly warned now not to eat certain kinds of fish because mercury in swordfish and bass, for example, is a potent neurotoxin that can cross the placenta and affect fetal brain and nervous-system development. Worried, Alfonso had her mercury level tested when she was pregnant in 2010. Fortunately, her results were normal, but for the first time, she began to wonder about the actual source of the mercury that contaminates fish.

The answer is air pollution. No, fish don't breathe air the way humans do. But what goes up must come down. Coal-fired power plants and other industrial sources release mercury into the air that eventually settles to the floor of lakes, rivers, and oceans. Bacteria in the water convert it to methylmercury, which diffuses into single-celled algae and then moves up the food chain, making its way into fish. People eventually eat the big fish, the ones that have the highest concentrations of mercury.

After her second baby was born, Alfonso dug around online for information about mercury. She learned that nearly half of the nation's miles of rivers and acres of lakes (including all of the Great Lakes) and 79 percent of its coastal waters were under a water-contamination advisory -- 80 percent of them due to mercury. (In Lake Erie, PCBs are the greatest concern.) She also found the Moms Clean Air Force, an advocacy group that organizes parents to take action on air pollution (see "The Power of Mothers," on page 3). Alfonso's eyes had been opened to the enormity of the larger problem. "Suddenly, those old jokes about perch didn't seem so funny anymore," she says.

We think of air pollution as belching smokestacks and hazy highways, and it is. But as the fish advisories prove, it can also be invisible. Seen or unseen, air pollution -- which results from gas and particle contaminants emitted by vehicles, planes, and factories -- can cause irreparable harm when it gets into our children's lungs and bloodstream. Kids' exposure to airborne pollutants is higher because they usually spend more time outdoors than adults do and they breathe faster. And because of their size, when children eat, drink, or breathe they have a larger exposure per pound of body weight than adults. They also breathe through their mouth more, so they lose the benefit of the filtering system in their nostrils. Toxins that are inhaled are far more dangerous than those that are ingested because they pass directly into the lungs and then the bloodstream, undiluted.

I recently moved back to the U.S. after living in Hong Kong, a city cloaked in a haze of pollution much of the time. At my children's school, administrators closely watched the daily pollutant levels and regularly restricted outdoor time. The Hong Kong government's health standards are outdated, but even following their conservative recommendations, pollution kept kids with asthma and other respiratory problems inside more than half the days in 2011. My family breathed a collective sigh of relief when we returned to New York, where the air is measurably better.

At one point, the United States looked more like Hong Kong -- there were days when a huge haze hung over Los Angeles or Pittsburgh. Any improvement we've made is thanks to 1970's Clean Air Act. "The nature of air pollution has changed," says Parents advisor Philip J. Landrigan, M.D., professor of pediatrics at the Mount Sinai School of Medicine, in New York City, and director of its Children's Environmental Health Center. "The truly gross pollution has gotten much better. But today we have a lot of fine-particle pollution, which is invisible but toxic to the lungs. So even though we've made progress, air pollution continues to be a threat to children's health."

Asthma tops a daunting list of physical problems associated with dirty air. What's more, researchers now think pollution doesn't just trigger asthma symptoms; it may cause the disease. Also on the list: allergies, sudden infant death syndrome, reduced birthweight, respiratory infections, and pediatric cancer. Unfortunately, even if we clean up our own skies, air pollution is truly a global issue. The fact is, the prevailing winds across the Pacific are currently blowing much of Asia's polluted air toward the United States.

Food Allergies: Helping Your Child Cope

Young Lungs

A child's lungs begin forming in utero and continue to grow well into adulthood. Like a seed that needs sunlight and water to flower, a developing baby's lungs need clean air to grow to their full capacity. "If they start breathing air that's polluted, the air sacs could stop growing," explains Kari Nadeau, M.D., Ph.D., an allergist and immunology specialist at Stanford University School of Medicine. In fact, a study of 1,900 Swedish children revealed a troubling finding: Exposure to air pollution from traffic in the first year of life was associated with a significant decrease in lung function among children up to age 8.

Asthma, an inflammation of the airways that results in less airflow to the lungs, affects about 7 million American children. It affects their parents too. "One of the scariest things is rushing your child to the emergency room because he's struggling to breathe," says Senator Kirsten Gillibrand (D-NY), whose 9-year-old son, Theodore, has asthma. The disease is clearly worse in children who live near highways. "There is overwhelming evidence now that any type of air pollution, especially ozone and diesel-exhaust particles, can make asthma worse," says Dr. Nadeau. She's among the researchers whose work has shown that children's asthma can be linked to air pollution because the toxic exposure alters the function of cells that regulate their immune system.

The problem can start before a child is born. A recent study at Seattle Children's Research Institute found that even low traffic-related pollution led to increased numbers of babies who were small for their gestational age. Christina Schwindt, M.D., an allergist and immunologist in Mission Viejo, California, is in the midst of a two-year retrospective study looking at pregnant women's exposure to pollution in the home, at work, and in their car, and the possible risk of their babies developing asthma. "We know there's a genetic basis to the increase in asthma rates, but the increase has been too quick to be due to genetics alone. We know it's genetics interacting with the environment," she says.

Air pollution may also affect the development of children's nervous and endocrine systems. Dr. Nadeau worries that when babies and young children inhale toxins, it could lead to cancer and other diseases later in life. Researchers haven't yet connected all of those dots -- and it isn't easy to do so given the number of variables involved -- but studies are underway looking at links between air pollution and obesity, autoimmune diseases, and cancer. Two recent studies, from UCLA and the University of Southern California, also found an association between prenatal exposure to traffic pollution and autism.

Policing the Air

The Clean Air Act required the Environmental Protection Agency (EPA) to establish and monitor safe air concentrations for six of the most common pollutants: ground-level ozone, particulate matter, sulfur dioxide, lead, carbon monoxide, and nitrogen dioxide. Between 1990 and 2010, levels of all six pollutants dropped and America's air quality improved as much as 80 percent depending on the pollutant being measured. Levels of lead, which can lower a child's IQ and damage the nervous system, fell the furthest -- 83 percent in the last 20 years -- after it was removed from gasoline.

The EPA is proud of its accomplishments. "In 2010 alone, the reductions we've already achieved under the Clean Air Act prevented 130,000 heart attacks, 86,000 hospital visits, 3.2 million lost school days, and 1.7 million asthma attacks or episodes," says Lisa Jackson, former EPA Administrator.

But a startling 42 percent of Americans -- which works out to nearly 132 million people -- live in counties where the levels of ground-level ozone and particulate matter make the air harmful to breathe, according to the American Lung Association. Ozone has been linked with premature deaths, and particulate matter in its smallest form (which is soot, also known as "fine-particle pollution") is what worries asthma experts the most.

The sources of air pollution vary across the country, since there are more coal-fired power plants in the East and the Midwest, for instance, and more cars in the West. Particulate matter can come straight out of a car's tailpipe, an industrial smokestack, or a woodstove chimney, or it can form in the atmosphere from chemical reactions of sulfur dioxide and nitrogen oxides. Ground-level ozone is created when nitrogen oxide and volatile organic compounds "cook" in the sunlight. (That's why levels are higher in warm summer months.) Whatever the source, particulate matter and ozone are harmful to health -- and children are among the groups considered at risk from exposure to these pollutants.

The most effective way to continue reducing pollutants is to stop them at the source. But that requires determination and political will. In December 2011, the EPA announced that power plants have to adhere to a new, separate set of standards for mercury and air toxics, but opponents are battling the new regulations in court. Even if those challenges aren't successful, the plants have until April 2015 to comply. Meanwhile, the EPA's tighter standards for fine-particulate matter, announced in December 2012, are also being fought; some members of Congress want to ease up the restrictions in the Clean Air Act because they believe the current regulations cost jobs. Warns Senator Gillibrand: "If the attacks on the Act were successful, polluters would be protected while the public health would be put at risk." But even if the opposition fails, states don't have to implement their plans to meet these new standards until 2020 (and can request a possible extension to 2025).

During my time in Hong Kong, the pollution was so pervasive that it was hard to know how I could protect my family. I could keep my windows shut, but most of the sources of pollution seemed far beyond my control. Here in the United States, I can make my opinions heard. That's what Gretchen Alfonso did. Now, she organizes events for Moms Clean Air Force as a national field manager, educating moms and kids and getting them to meet with politicians. As she puts it, "It's so important for us to realize we have a voice."

The Power of Mothers

In 2011, together with the Environmental Defense Fund, mom of two Dominique Browning founded Moms Clean Air Force: bloggers, field organizers, and parents (dads are welcome!) working to protect the air for children. The idea arose when Browning, the former editor-in-chief of House & Garden, started writing about issues like climate change. "Nobody was talking to people who don't think of themselves as environmentalists but who care deeply about kids' health," she says.

Her goal is to make that connection and to harness the considerable political power of moms' advocacy efforts. MCAF has more than 135,000 members already; their activities encompass education, letter-writing campaigns, and testimony in Washington. MomsCleanAirForce.org provides multiple ways parents can take action. For instance, you can write to President Obama or the Environmental Protection Agency and urge them to take steps to clean our air.

What You Can Do

Get the air-quality forecast with an app from airnow.gov or lung.org, or a daily e-mail from enviroflash.info.

Don't let kids -- especially those with asthma or other lung diseases -- play near high-traffic areas, or anywhere outside when pollution is high.

Watch for warning signs of asthma. Kids may wheeze, cough, or just not be able to keep up with other kids when they're running around. Those with bad allergies or eczema may be particularly at risk for asthma.

Drive with your windows closed on busy highways, in high-pollution areas, and on days when the air quality is low. Don't use vents that circulate outdoor air, either.

Don't burn trash, since the smoke releases toxic pollutants into the air.

Encourage your school to stop buses from idling.

Walk, bike, or carpool to cut down on auto emissions.

Use hand- or electric-powered lawn mowers. Gas-mower engines often have no pollution-control device.

Fill up your car after dark. Sunlight acts on the gasoline that evaporates in the air to cause ozone.

Get involved. One big issue to weigh in on: a new set of emissions standards for vehicles (Tier 3 standards), which could reduce pollutants by as much as a third. This proposed set of rules needs support; momscleanairforce.org/action provides more info.